Ceramic honeycomb structures are known in the prior art. Such structures generally comprise a plurality of interconnected web walls that form a matrix of gas-conducting cells which are typically square or hexagonal in shape, and a cylindrical outer skin surrounding the cell matrix. The outer edges of the matrix of web walls is integrally joined to the inner edge of the outer skin to form a single, unitary structure, which is usually cylindrical in shape.
Such ceramic honeycomb structures find particular application as either particulate filters in diesel exhaust systems, or as catalyst substrates for automobile exhaust systems. Hence these structures have an inlet end for receiving exhaust gases, and an outlet end for expelling these gases. Ceramic honeycomb structures used as diesel particulate filters typically have a cell density of between 100 and 400 cells per square inch, and webs on the order of 12-20 mils thick. The inlets and outlets of the matrix of gas-conducting cells are plugged in a “checkerboard” pattern on the inlet and outlet ends of the structure to force the diesel exhaust gases through the porous ceramic material forming the web walls, thereby filtering out the particulate soot generated within the exhaust of diesel engines. In order to maintain the gas-permeability of such a honeycomb structure, it is necessary to periodically burn-off the particulate material that accumulates on the inlet-side of the webs forming the gas-conducting cells. Hence, the inlet cells are periodically exposed to a hot flame in a “burnout cycle” designed to ablate the accumulated particles of soot. The central webs of a ceramic honeycomb structure used as a diesel particulate filter may be raised to a temperature of 1100° C. during such a burn-out cycle, while the outer skin is heated to only about 500° C. The resulting 500+° C. thermal gradient creates thermal stresses in the ceramic honeycomb that can cause cracks and other discontinuities, primarily in the outermost cells which contact the inner edge of the outer skin.
When such ceramic honeycomb structures are used as ceramic catalyst substrates, the cells are not plugged as with diesel particulate filters and gases are allowed to pass straight through the gas-conducting cells. The cell density is made higher (i.e., about 300-900 cells per square inch) in order to maximize the area contact between the automotive exhaust gases which blow directly through the gas conducting cells, and the web walls. To reduce the pressure drop that the exhaust gases create when flowing through the honeycomb structure, the web walls are rendered thinner than in structures used for diesel particulate filters, i.e. on the order of 2-6 mils. The use of such thinner walls further advantageously reduces the light-off time (i.e., the time it takes before the webs reach the required 250° C. before the catalyst impregnated within the web walls begins to effectively remove NOx and other unwanted pollutants from the exhaust gases). The frequent rapid heating of such structures from ambient temperature to 250° C. whenever the automotive vehicle is started likewise generates a substantial thermal gradient across the diameter of the honeycomb structure. These thermally induced stresses are maximized at the interface between the thin web walls and the outer skin of the honeycomb structure.
In both the cases where a ceramic honeycomb structure is used as diesel particulate filter, or as a catalytic substrate, the applicants have observed that the thermally induced stresses occurring at the interface between the cell matrix and the inner edge of the outer skin are exacerbated by the frequently oblique orientation between the web walls, and the outer skin. Such an oblique orientation is a result of the imposition of a circular or rounded outer skin around a matrix of square or hexagonal cells, which necessarily causes some of the web walls to join the outer skin at an angle, such as of 45° and less. To solve these problems, several honeycomb structures employing a combination of radial and tangential webs have been proposed in the prior art. The advantages of such designs are the elimination of webs on the outer edges of the honeycomb matrix that join the inner edge of the outer skin at oblique (non-orthogonal) angles. The resulting substantially orthogonal orientation between the outer edges of the radial web walls and the inner edge of the outer skin reduces the stresses produced by heat gradients. However, such known radial-web designs include (1) “wagon wheel” configurations having an interior portion formed from a matrix of square cells, and a peripheral portion formed from a single, tangential layer of radial cells is created between a single, cylindrical wall and a plurality of short, radially oriented webs that form the side walls of the cells, or (2) a stacked radial cell configuration wherein each of the radial webs extends substantially the length of the radius between the centroid and the inner edge of the outer skin, or (3) an imbricated radial cell configuration cell configuration having rings of staggered radial cells where each radial web extends only the length of a ring of cells.
Unfortunately, the applicants have found that all of these designs have shortcomings. While the “wagon wheel” design has been found to ameliorate the stress problem between the cell webs and the outer skin, it tends to transfer these stresses to the interface between the outer edges of the cell matrix and the inner edge of the cylindrical wall that forms the inner wall of the ring of radial cells. While the second radial design employing radius-length walls avoids the stress or displacement problem associated with the “wagon wheel” design, it inherently creates an unacceptably high cell density near the center of the cell matrix, where the radial webs simultaneously converge. This, in turn, generates an unacceptably high pressure drop across the honeycomb structure. Additionally, such a structure is difficult, if not impossible to manufacture via conventional extrusion techniques, as the convergence of the web walls at the center of the matrix creates disparities in the flow rate of extruded ceramic material that in turn distort or weaken the final structure. While the imbricated radial cell design solves the aforementioned high cell density and manufacturing problems, it is unacceptably weaker in its interior than conventional designs using square or hexagonal cells.
Accordingly, there is a need for a radial cell ceramic honeycomb structure that maintains the stress-reducing advantages associated with an orthogonal interface between the outer web walls of the cell matrix and the inner edge of the outer skin, but avoids the stress-displacement, high cell density and interior weakness problems associated with prior art radial cell designs. Ideally, such a radial cell honeycomb structure would either maintain a desired cell density across the diameter of the honeycomb structure, or reduce the density of the cells near the outer perimeter of the structure to promote hot gas flow more toward the periphery, thereby reducing the thermal gradient and hence thermal stresses. It would be desirable if such a radial cell honeycomb had improved compressive strength to better withstand the exterior stresses applied to such structures during the manufacturing process. Finally, such a structure should also have improved strength for handling the stresses that occur as a result of the heat up and cool down cycles of the honeycomb structure which occurs after the burnout cycle in a diesel particulate filter, or engine start up and shut-off in a catalytic carrier.